When designing high output marine propulsion machineries, the reversing has hitherto been a limiting factor. Certain engine types of prime movers are designed to be directly reversible so they can supply the required output in both directions of rotation. Other machineries, as for instance turbines, do not possess such properties, while, with a third type, reversing is possible, constructional and operational complications make it more convenient to reverse by other means than the power unit itself.
Many gears with reversing facilities are very expensive, and have, for that reason, mainly been installed in warships. With steam turbines, the reversing problem has been solved by providing separate astern-turbines. Reversible gears have been proposed, in which the torque is transferred to different combinations of gears by means of hydraulic clutches being filled and emptied, and in turn connecting the machinery to the propeller via different gearways.
It has also been proposed, during a reversing operation, to reduce the torque acting upon the gear to about zero, by means of brakes mounted at the propeller shaft. As long as the ship is moving through the water, the propeller must also be braked which imposes a heavy load upon the brake.